Most PAH substances are derived from incomplete combustion, coke production and tar processing as well as crude oil processing and storage. They are found in high concentrations at sites of former or present industrial activity. PAHs typically comprise several condensed aromatic rings (benzene) of varying physical-chemical characteristics and toxicity. The USEPA has identified 16 PAH congeneres as being the most hazardous ones. Due to their formation process, these 16 PAHs usually occur together with a large number of co-contaminants (hydrocarbons, alkylated and substituted aromatics, etc.) that have been considered as being of reduced risk. PAH compounds can become environmental contaminants in soils, with implications for human and environmental health, and may remain in the environment for extended periods. The level and type of PAH contamination is difficult, if not impossible, to determine by odour or visually unless gross contamination has occurred, so that rapid and inexpensive instrumental methods for the prediction of PAH compounds in soils is desirable for the identification of such hazards.
Urban areas with sites predominantly industrial in nature are increasingly being replaced by residential development, many of these sites, however, have been subjected to contamination from oils, fuels and other hydrocarbons, such as PAHs, and are therefore unsuitable for residential development because of failure to meet with environmental protection guidelines for acceptable concentrations of hazardous compounds. These contaminants can often be remediated to acceptable levels e.g. by removing contaminated soil but this process can incur significant costs and furthermore, delays caused by lengthy conventional PAH analysis in laboratories.
Current testing typically involves the extraction and instrumental analysis of the contaminant components in the soil extracts. Testing of PAHs in soils is usually carried out using high pressure liquid chromatography (HPLC) via solvent extraction of the PAH components from the soil samples, or by gas chromatography—mass spectroscopy (GC-MS), particularly for the more volatile PAH components, where analysis can take up to 40 minutes per run. This can be a major disadvantage of conventional analysis, considering the time and effort required for extraction and calibration, where a number of hours may be required for a result with chromatography.
A rapid, cost effective and potentially in-field method for predicting PAH concentration in a site would provide significant advantages in meeting the needs for contaminant quantification and monitoring. Infrared spectrometry, through the use of regression models, could offer a possible alternative approach for the rapid analysis of soil contaminants. Infrared spectroscopy distinguishes between chemical compounds by detecting the specific vibrational frequencies of molecular bonds, producing a unique infrared “spectral signature” thus enabling its identification and quantification. Whilst most existing infrared applications used to characterise hydrocarbon contaminants measure the spectra of infrared radiation transmitted through a sample cell containing a sample extract, considerable advantages can be achieved with reflection of the infrared radiation directly from the soil sample surface. One such method, diffuse reflectance infrared spectrometry in the mid-infrared (MIR) and visible-near infrared (vis-NIR), when coupled with multivariate chemometrics regression techniques such as partial least-squares (PLS), has been used for the rapid analysis of agricultural soil analysis for a large number of soil chemical and physical properties (Janik et al. 1998 Aust. J. Exp. Agric. 38:681-696; Reeves et al. 1999 Journal of Near Infrared Spectroscopy 7(3):179-193; and Cozzolino and Moron 2003 Journal of Agricultural Science. Volume: 140 Pages: 65-71 Part: Part 1). When using Fourier Transform based infrared spectrometers the method of diffuse reflectance is called DRIFT.
Madari et al. describe the mid- and near infrared spectroscopic assessment of soil compositional parameters and structural indices in two Ferralsols (J Geoderma 136(1-2) 245-259 (2006)). Bulk soil samples were analysed, e.g. for soil organic carbon. Absorption bands of carbon in organic bonds were found in the Mid-IR spectral range.
Infrared diffuse reflectance spectroscopy has been used to develop diagnostic screening tests for PAHs in soils using vis-NIR and an ordinal logistic regression method (Bray et al., Australian Journal of Soil Research 2009, 47:433-442). The above method differed from the present invention in that only threshold level classes of these soil contaminants were predicted, and that the regression models were based only on the loose correlation between soil composition and PAH concentration, rather than specific PAH peaks identified in the samples. Direct detection of PAH spectral peaks from NIR spectroscopy was not made by these authors and formal MIR analysis of PAHs was not carried out.
There are two potential problems with using diffuse reflectance infrared spectrometry for soil contaminant analysis. Some of the spectral peaks typically attributable to PAHs may occur in frequency regions having overlap with naturally-occurring soil organic matter (NOM), also called soil organic matter (SOM) or soil organic carbon (SOC), or with certain soil minerals. Also, due to the low concentrations usually encountered in contaminated samples, the sensitivity of the infrared for PAH predictions may be insufficient for accurate quantitative purposes.
It was the object of the present invention to provide a simple and rapid method with sufficient accuracy to determine PAH concentration in solid samples including soils.